This Datacron unlocks the codex entry - Galactic History 13: The First Great Schism
This datacron holds unheard of power and knowledge collected by an ancient race. You access its power and discover writings which are clearly only one small piece of a massive galactic history:
After the Force Wars, when the Jedi Order learned of the destructive power of the dark side, the Jedi devoted themselves to the path of the light. In the early days of the Republic, a young Kashi Mer Jedi named Xendor defied the order's code and began experimenting with dark side techniques. Incensed, the Jedi Order confronted Xendor and banished him from Ossus.
Xendor, now immensely powerful and full of rage, raised an army on the planet Lettow and led it against the Jedi. Not since the Force Wars had the Jedi fought their own. Xendor and the Legions of Lettow eventually fell in battle; his lover, Arden Lyn, was defeated by the Jedi Master Awdrysta Pina and placed into a strange stasis, sustained by the dark side. Although this first schism within the Jedi was over, it would not be the last.
This Datacron is found on Ord Mantell at the coordinates -977, 9. That is on the northern beach of Mannett Point, for those not good with Coordinates. What you need to do in order to reach this Datacron is travel around the western end of the mountain (don't go inside, instead drop down off of the path and follow the beach around until you find the little divet on the map. This is where the Datacron is located. Honestly there is no reason for a super long explanation so here are a few pictures.